The present invention relates to an improved process and reactor system for the hydroxylation of benzene or derivatives thereof to form phenol or related compounds.
Phenol or a derivative thereof can be produced by a single-step oxidative hydroxylation of benzene or a derivative thereof, using nitrous oxide over a catalyst. For example, PCT publication WO 95/27560 describes such a process in which benzene is combined with nitrous oxide over a zeolite catalyst that has been hydrothermally treated, converting the benzene to phenol. It is believed that specific active sites on the zeolite, referred to as .alpha.-sites, are responsible for the desirable conversion in such a process. After a batch of catalyst is used for a time in the process, some of the .alpha.-sites become deactivated, and the catalyst must then be reactivated.
In prior art processes of the general type described above, reactivation of the zeolite catalyst has been done in situ with an oxidant activator such as nitrous oxide. However, this presents several problems. For example, the intermediates and products of the hydroxylation reaction are generally more reactive than the reactant. This often leads to production of undesirable over-oxidated byproducts instead of maximum production of the desired end product. Moreover, the free oxidant used to activate the .alpha.-sites is generally less selective than the .alpha.-sites, which also contributes to increased production of undesirable byproducts. Another problem is that explosive mixtures can sometimes be formed between the reactant and the free oxidant activator, which can present a safety problem.
These problems constrain the practical operation of a commercial process. In general, in situ reactivation of the catalyst leads to a compromise choice of process conditions because the preferred conditions for activating the .alpha.-sites may not be the optimum conditions for the hydroxylation reaction. In particular, with in situ activation of .alpha.-sites, reaction conditions must be sought that minimize raw material, energy, and capital consumption while forming a safe and commercially viable operation. Inevitably this involves a compromise among these three major cost factors. For example, to maintain operation outside of flammability regions, inert diluents are often added that must eventually be separated and recycled, requiring additional capital and energy. Also, because of the nonselective nature of the oxidant activator, the reactions are run at relatively low reactant conversion to minimize over-oxidation to undesired byproducts. This results in a compromise between raw material cost and energy usage for recycle.
A need exists for processes and catalysts having improved performance, so that the conversion of an aromatic hydrocarbon such as benzene to phenol or another desired product can be made more economical. One potential way of improving catalyst performance is by selective introduction of sites for hydroxylation, for example by introducing iron into the catalyst. However, introduction of active iron via synthesis can present complications.